SOFIA, March 3 (Reuters) - Tens of thousands of Bulgariansangry over poverty and corruption protested in more than a dozencities on Sunday, as a lack of clear support for any politicalparty mired the country in limbo days after the government wastoppled.

Prime Minister Boiko Borisov quit along with hiscentre-right government on Wednesday after two weeks ofsometimes violent protests. He remains in office until aninterim government is appointed, most likely next week, whichwill take Bulgaria to elections due on May 12.

However Bulgarians are still struggling to unite behind asingle political leader or give voice to a clear set of demands.

Polls suggest neither Borisov's rightist GERB party nor theopposition Socialist Party has enough support for an overallmajority, and whichever wins the election will have to try toassemble a coalition to form a working government.

Thousands of people took to the streets of cities includingthe capital Sofia, Plovdiv, Burgas, Blagoevgrad, Ruse and Slivenon Sunday - a national holiday that marks the 135th anniversaryof Bulgaria's liberation from Ottoman rule.

In the biggest rally, about 50,000 protested in the BlackSea city of Varna, local media reported.

"It is obvious that the protesters are not united and thiscould very quickly destroy the enthusiasm of the people," saidGeorgi Trendafilov, a demonstrator in Sofia downtown.

Borisov was hospitalised with high blood pressure on Sundayfor a second time this week.

Following a three-day spell at hospital, doctors haveadvised the former bodyguard and karate black belt to take fullrest and refrain from sports. The outgoing prime minister playsfor a third division soccer team, Vitosha Bistritsa.

Hospital officials said he was admitted with hypertension atlunch time and it was too early to say when he would bedischarged.

Six years after joining the European Union, Bulgaria trailsfar behind other members. Its justice system is subject tospecial monitoring by Brussels and it is excluded from thepassport-free Schengen zone because of other members' concernsabout graft.

The country's public debt is one of the lowest in the bloc.But business cartels, corruption and wages that are less thanhalf the EU average have kept many from feeling the benefit.

It also has the cheapest electricity costs in the EU but anincrease in prices since last July under an energy marketliberalisation has made it harder for Bulgarians to heat theirhomes through a cold winter.

POWER PROTESTS

The demonstrations began with a handful of youngstersprotesting against high electricity bills. Eventually, hundredsof thousands of Bulgarians took to the streets, angered by theirlow living standards.

President Rosen Plevneliev said an interim government wouldaim for stability by sticking to the 2013 budget, which foreseesa deficit of 1.3 percent of GDP, and implementing previouscommitments such as a 9 percent increase in pensions from April.

He also said he would set up a 35-member public council toadvise the interim government and represent the people'sinterests. But consultations for the establishment of thecouncil at the presidency collapsed on Saturday.

Representatives of protesters, objecting to the inclusion ofsome wealthy businessmen, walked out of the talks. They saidthey could not "sit at the same table with those they werefighting".

"We are going out to fight until the end, we will notnegotiate with oligarchs," said Angel Slavchev, one of theleaders of the demonstrations. A trade union leader also quit,objecting to the composition of the council.

Earlier this week, Borisov dismissed the idea of a governingnational unity coalition.

Support for Borisov's rightist GERB party has fallen overthe last year, and it is now neck-and-neck with the oppositionSocialist Party.

Just before resigning, Borisov had proposed to cutelectricity prices by 8 percent and alarmed investors by sayingthe government would revoke the distribution licence of theCzech utility CEZ, risking a diplomatic row with theCzech Republic and EU.

The energy regulator proposed a smaller, 6.4 percent cut onFriday, a few days after CEZ and the other two distributors,Austria's EVN and the Czech firm Energo-Pro,said they had done nothing wrong.